Syria risks a "catastrophic civil war" in the wake of the massacre of more than 100 civilians, including 49 children, the UN secretary general, Ban Ki-moon, warned has warned as the Damascus government again blamed the killings on terrorist gangs.

Amid evidence of international disarray over how to handle the crisis, the UN chief bluntly told President Bashar al-Assad to implement the six-point peace plan being promoted by Kofi Annan, though many consider it already effectively dead in the face of rising violence.

"I demand that the government of Syria act on its commitment to the Annan peace plan," Ban told a conference in Turkey. "The massacres of the sort seen last weekend could plunge Syria into a catastrophic civil war, a civil war from which the country would never recover."

UN monitors had not been sent to Syria "just to bear witness to the slaughter of innocents," he said. "We are not there to play the role of passive observer to unspeakable atrocities."

Syria said "preliminary" findings of it own investigation showed the massacre in the Houla area near Homs was the work of 600-800 "armed men".

Brigadier-General Qasim Jamal Suleiman, who is leading the inquiry, said the army had not been present in the area when the killings were carried out last Friday. The victims had refused to join anti-regime demonstrations and some bodies were of armed men killed in clashes, he said. None had been killed by shellfire.

Jihad Makdissi, the Syrian foreign ministry spokesman, said: "The massacre is in line with the armed opposition's pathetic plan to show that there is sectarian strife."

The UN human rights council in Geneva is to debate the issue on Friday amid calls for an independent UN-led investigation into the worst massacre so far in Syria's 14-month uprising – the most violent chapter of the Arab spring.

The Syrian government also announced on Thursday that it had released 500 prisoners "without blood on their hands" – an apparent gesture to Annan after his talks with Assad this week. But many thousands more remain in detention and Damascus has failed to implement nearly all the other elements of the UN-Arab League plan.

Makdissi repeated that Syria wanted the plan to succeed and urged opposition groups who were against foreign intervention to enter talks with the government.

As Washington and Moscow swapped accusations, analysts said that with Russian backing apparently guaranteed, Assad feels that he can ride out international outrage over the Houla killings, even though 15 countries have expelled Syrian ambassadors and diplomats.

Support from Moscow and Beijing was the main item reported by Sana, Syria's state news agency, for the second day running. "Russia's position … is balanced and consistent and completely logical," said Dmitry Peskov, spokesman for President Vladmir Putin. "So it is hardly appropriate to talk about this position changing under someone's pressure."

On Wednesday, the White House accused Russia and China of being on "the wrong side of history". Susan Rice, the US ambassador to the UN, hinted at action "outside of the Annan plan and the authority of this [the UN security] council if there was no agreement inside it". It looks unlikely that there will be. Hague said Britain and its EU partners would encourage other countries to impose sanctions on Syria without UN approval.

Signs of divisions emerged from within the ranks of the armed wing of the Syrian opposition, with fighters on the ground repeating a warning to the Assad regime to implement the terms of the Annan plan by noon on Friday or the ceasefire – in truth more notional than real for the past six weeks – would end.

The Free Syrian Army's Turkey-based commander, Colonel Riad al-Asaad, earlier denied any such deadline had been issued – presumably in the hope of pinning the blame for failure entirely on Damascus.

"Nobody has the right to issue press releases, take decisions, or speak about operations in the Free Syrian Army's name, except for the FSA command inside Syria," spokesman Colonel Kassem Saadeddine told AFP. "From now on, all decisions will be taken from inside Syria. Anyone who wants to speak in the name of the FSA should do it from the battlefield, not through media. We are the ones leading the operations; we are the ones mobilising the street.

"It is our children who are being massacred, not the children of those staying in hotels and in [defectors'] camps," he added, in reference to Syrian army deserters who sought refuge in Turkey.

The rift highlighted concerns about the ability of the armed opposition to act in a disciplined manner and under central political control as the international debate continues about how to handle the crisis.